Semiconductor injection lasers are used as light sources in long-haul optical fiber transmission system where the information which is transmitted is in the form of an encoded light beam which propagates through a glass fiber. The encoded light beam is generated by modulating the current injected into the laser. For error-free transmission, the intensity of the transmitted light beam must be maintained within a specific allowable range. This is usually accomplished by measuring the power output of the laser with a monitoring photodetector which is physically separate from the laser. The monitoring detector measures the average power output of the laser. The signal from the monitoring detector, acting through a feedback circuit, adjusts the injection current of the laser to maintain the average power output of the laser within the allowable range. This adjustment is necessary because the output power may change slowly during operation as a result of slow degradation and/or as a result of variations of the ambient temperature.
Currently, the output power of a laser is measured by diverting a portion of the generated optical power to a separate optical detector. While a separate detector is adequate, it is desirable to have a photodetector which is integrated with the semiconductor injection laser in the same semiconductor chip. Integration of a laser with a photodetector on the same chip can result in a compact laser-photodetector device which has a reduced size, operates with increased speed, provides high performance and has good reliability--all at a lower cost.